fs/9p: allow disable of xattr support on mount

xattr creates a lot of additional messages for 9p in
the current implementation.  This allows users to
conditionalize xattr support on 9p mount if they
are on a connection with bad latency.  Using this
flag is also useful when debugging other aspects
of 9p as it reduces the noise in the trace files.

Signed-off-by: Eric Van Hensbergen <ericvh@kernel.org>
Reviewed-by: Dominique Martinet <asmadeus@codewreck.org>